Emissions from internal combustion engines, including diesel engines, are limited by legislation put in place by governments worldwide. Original equipment manufacturers (OEMs) are seeking to meet these legislated requirements through a combination of engine design and exhaust gas aftertreatment. The exhaust systems used to carry out exhaust gas aftertreatment commonly comprise a series of catalysts and/or filters that are designed to carry out certain reactions that reduce the proportion of exhaust gas species limited by such legislation. Exhaust gas species limited by legislation include nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM).
One exhaust system component for use in treating such exhaust gas species is the particulate filter substrate. Typically, PM trapped in the filter is combusted either actively or passively. One form of passive combustion is to combust the trapped PM in nitrogen dioxide as described in our EP 341832. PM combustion in nitrogen dioxide takes place at temperatures substantially lower than in oxygen (less than 400° C. compared with >550° C.). A convenient way of generating nitrogen dioxide is to oxidise nitrogen oxide in the exhaust gas on a suitable oxidation catalyst disposed upstream of the filter. A device of this nature is marketed by Johnson Matthey as the continuously regenerating trap or CRT®.
One form of active filter regeneration is intermittently to introduce additional hydrocarbon fuel into the exhaust gas and to combust this in order to increase the filter temperature. Combustion of the additional hydrocarbon fuel can be effected on the filter itself by coating the filter with a suitable combustion-promoting catalyst. A suitably catalysed filter is often referred to as a catalysed soot filter or CSF.
During active regeneration the CSF may need to reach temperatures of approximately 600° C. to permit PM to be removed (combusted) at a sufficient rate. However, if during an active regeneration event, a period of low exhaust gas flow occurs, e.g. when the engine/vehicle is caused to idle, the reduced gas flow prevents heat from being removed from the CSF. This can result in parts of the filter reaching temperatures in excess of 1000° C. (see FIG. 1—note that the top of the Figure is the CSF inlet and the bottom of the Figure is the outlet; lighter shades indicate higher temperatures). Such high temperatures can cause two major problems. Firstly, the catalyst can sinter, reducing its surface area and as a consequence catalyst activity is lost. Secondly, high thermal gradients can occur in the substrate leading to mechanical stress caused by differences in thermal expansion. Under extreme conditions the thermal gradients and stresses can cause substrates to crack thereby resulting in a failure of the integrity of the CSF. Therefore, the challenge is in controlling the active regeneration of the CSF so that it can reach temperatures sufficiently high to remove PM but not so high as detrimentally to cause damage to the catalyst and/or the filter substrate.
In order to prevent the filter from reaching such damagingly high temperatures, a heavier filter substrate can be selected. A temperature change in the filter substrate can be represented by equation (1), assuming a quasi-adiabatic system:ΔT=bulk volumetric heat capacity×Q/mass of the filter  (1)where coefficient Q is proportional to the mass of soot on the filter.
It follows that by increasing the mass of the filter, ΔT is reduced.
However, increasing the mass of, e.g. a cordierite wall flow filter, results in the material containing fewer pores and this in turn undesirably increases back pressure in the system. Increased back pressure results in increased fuel consumption and potentially the necessity for more frequent active regenerations.
It is known from U.S. Pat. No. 6,827,909 B1 to increase the thermal mass of a flow-through monolith substrate by coating a downstream zone thereof with a thicker washcoat so that it can “store” heat for operating conditions that produce lower exhaust gas temperatures, thus maintaining the activity of a catalyst loaded on the monolith substrate during such temperature conditions. The upstream zone has a relatively lower thermal mass, which allows it to reach active temperature more quickly. However, the thicker washcoat can undesirably increase the backpressure in the system as observed in EP 1379322.
We have now devised a means of selectively increasing the mass of a filter substrate without increasing the back pressure to the extent observed in higher mass filter substrates or the use of thicker washcoats.